Light Groups support

[boberfly]

Just adding the ticket here also, we will definitely need this merged in: https://developer.blender.org/D3607

I believe the outstanding issue is that adaptive sampling wasn't compatible with it. This might just require your eyes on it @skwerner

Once this is done I can look into implementing it into hdcycles & add the necessary schema to the lights and geometry tags to specify the name to match the AOV name.

Cheers!